1. Field of the Invention
This invention relates to the art of electrochemical cells, and more particularly to a new and improved electrode covering for alkali metal-halogen and alkali metal-oxyhalide cells.
The recent growth in small-sized electronic devices has required electrochemical cells having high energy density and current delivery capability. As a result of this need, alkali metal-halogen and alkali metal-oxyhalide cells have been developed wherein the anode typically is lithium and the electrolyte in operative contact with the anode and the cathode element is respectively solid or liquid. In the development of such cells it has been found advantageous to coat the operative electrode surfaces, and particularly the anode surface with organic electron donor material. For example, in alkali metal-halogen, solid electrolyte cells, such an electrode covering beneficially results in a greater utilization of the lithium anode and a reduction in cell impedance. In alkali metal-oxyhalide, liquid electrolyte cells, an organic electron donor material electrode covering advantageously inhibits the formation of compounds on the anode surface that otherwise would cause passivation giving rise to a voltage delay after storage at elevated temperatures.
2. Discussion of the Prior Art
The prior art has developed a variety of techniques for applying coatings including painting or brushing a solution of organic electron donor material dissolved in solvent onto the anode surface. Another method comprises forming a sheet of the organic electron donor material such as by hot pressing and then applying the preformed sheet onto the anode surface, often in conjunction with an adhesive. These methods are well known to those of ordinary skill in the art. However, they are often time consuming and do not necessarily result in a uniform layer of organic material being applied to the anode surface.
As a solution, U.S. Pat. No. 4,812,376, issued Mar. 14, 1989 to Rudolph and entitled Anode Coating For Lithium Cell, which is assigned to the assignee of the present invention and is incorporated herein by reference, discloses an improved anode coating for use in an alkali metal-halogen or oxyhalide electrochemical cell comprising a film of synthetic open mesh, non-woven fabric or cloth material having the organic electron donor material impregnated therein. The film is prepared by contacting the non-woven fabric material with a solution of the organic material and solvent, followed by drying. The resulting film is flexible enough to be applied to the operative surface of the anode by pressing, thereby coating the anode. The open mesh, non-woven fabric material used as the substrate can comprise a polyester open mesh material, such as #3251 Hollyrex-Paper polyester. This material works well, but the non-woven and fabric characteristics cause the substrate to vary in its weight per unit area, i.e., large standard deviation, resulting in a non-uniform layer of organic material being contacted with the anode surface. Alternate spin-bonded non-woven or woven fabric substrate materials are known, but they too vary widely in their unit weight.
The present invention thus provides in an alkali metal-halogen or oxyhalide electrochemical cell wherein an alkali metal anode, preferably lithium, has a surface in operative contact with a halogen-containing or oxyhalide cathode/electrolyte including a solvent if necessary, a covering on at least one of the electrodes comprising the cell and more particularly a covering on the anode surface. The present covering is nonfabric and begins as a continuous and solid film of substrate material having a uniform unit weight. The solid film is perforated to provide for ion flow therethrough and coated with organic material, for example organic electron donor material, or other suitable coating material. The perforated film substrate material is flexible, preferably comprising a mechanically perforated synthetic polyester film material, and the film is prepared by contacting the substrate material with a solution of the organic material and solvent, followed by drying. The resulting coated film is flexible and is applied to the operative surface of the electrode preferably by pressing, thereby covering the same. The flexible film can be applied equally well to electrode surfaces which are either smooth and flat or irregular.